The present invention relates to an apparatus for detection of a focused condition by projecting a definitely-shaped light beam to the object and examining photometrically the light beam reflected from the object with light-receiving means arranged at a position corresponding to a definite focusing position, thereby examining whether or not the focusing optical system is in a focused condition. Generally, in the viewing optical apparatus or optical camera such as endoscopes, photographic cameras and TV cameras etc., there are widely employed means for detection of a focused condition for examining whether or not such a condition is obtained so that the image produced by the optical system for photographing or focusing for the optical distance from the object may be clearly focused whereby the object may be clearly photographed or observed. Occasionally there may be additionally arranged a means which can displace said photographic optical system automatically to the position of a focused condition.
In the case of the object itself being dark or a sufficient photographic optical system being employed, the conventional apparatus for detection of a focused condition provide a generally smaller power output by their light-receiving element to make the focus-detection substantially impossible.
An irradiation means for directing the definite shaped light beam toward the object is described in Japanese Patent Publication No. 19,810/74. The Patent employs an optical system independent from the photographic lens system as an irradiation means. The optical system is not applicable to an endoscope, etc. that must photograph through a single light path.
In the means for employing a split prism as described in Japanese Patent Application Disclosure No. 128,923/81, when the detection apparatus is demanded to have an accuracy higher than a certain accuracy level, plural light-receiving elements are required. Accordingly the support system, for examining whether or not proper focusing is obtained by comparison with output signals, is rather complicated and particularly in limited production of smaller quantities such means may be high in cost.
Furthermore, Japanese Patent Application Disclosure No. 125,713/81, discloses apparatus for detection of a focused condition capable of determining whether an optical system is well-focused or not, even in the case of the object being faintly illuminated and the photographic optical system being dark wherein a flickering light is projected from its source to the object and the power-output signal of an extinguished section is subtracted from the power-output signal of a lighted section to alleviate the influence of outside light other than said light source. However, in addition to the definite focused surface of the film etc., two locations are arranged on both sides of said focused surface. The light projected to the object by the projection means is conducted to one of said locations. In this conventional example, relatively large surfaces are necessary for the lighting means and the light-receiving means and the structures of the optical system and the circuit system for detecting the focused condition are complicated, so that the detection apparatus is expensive.
On the other hand, when a conventional apparatus for detection of a focused condition is employed with an endoscope to detect the focusing condition, the light reflected from the object, such as organs inside the cavity of a living body, has lower contrast and also the viewing optical system of the endoscope has a faintly lighted sight. Also, it is often necessary to photograph from a short distance and so the focus depth becomes so small that both the apparatus for detection of a focused condition using the contrast method and the apparatus for setting the focused condition for automatically establishing a focused condition with such apparatus for detection of a focused condition are poor in reliability.
Accordingly, there is a requirement for a reliable and capable apparatus for detecting a focused condition which is effective even with an object of optically lower contrast such as living organs in a body cavity.